Check out this awesome clock by Che-Wei Wang and Taylor Levy:
Thermochromic Clock is a 4-digit 7-segment timepiece. Each segment in the display is made with a length of nichrome wire and then covered by a thick layer of black thermochromic paint. Time is displayed by applying voltage to the nichrome wire. As the wire sustains an electric current, it heats up the surrounding thermochromic paint, causing it to become transparent.